1. Field of the Invention
The present invention relates to electronic devices, i.e., electrical products based on the technology of electronics. Electronic devices relating to the present invention include not only a typical electronic device having electronic components arranged on a substrate but also other types of electronic devices including a solar cell, a photovoltaic unit and a light-emitting device, a lighting apparatus and a signal light using a light-emitting diode.
2. Description of the Related Art
In electronic devices, as is well known, traces having a predetermined pattern are formed on one side of a substrate and electronic components such as active components or passive components are soldered onto the trace pattern. The trace pattern can be obtained by applying an etching resist to a Cu foil previously formed on the substrate and then patterning the Cu foil by photolithography. A solder resist film of a thermosetting epoxy resin or the like is formed on the substrate such that the Cu foil is exposed only at portions of the trace pattern which are required for soldering of the electronic components while solder cannot adhere to the other portions which are not required for soldering. Then, the electronic components are soldered to the exposed Cu foil.
As described above, the production of conventional electronic devices requires many processes, e.g., preparation of a copper-foiled substrate, application of an etching resist, photolithography, application of a solder resist and mounting of components. Thus, there is a limit to cost reduction and improvement in productivity.
The above process may be replaced with a process of screen-printing a conductive paste directly on the substrate. In this case, the conductive past to be used should have metal or alloy powder dispersed in an organic vehicle as a conductive component. The organic vehicle comprises an insulating resin such as a thermosetting insulating resin or a thermoplastic insulating resin and a solvent. If necessary, a third component may be added so as to improve dispersibility of the metal powder or ensure fire retardancy.
The trace pattern thus obtained has the metal powder dispersed in the insulating resin. This deteriorates the conductivity as compared with the case of a sheer metal conductor.
In order to improve the conductivity, accordingly, the packing ratio can be increased by using metal powder having a small grain size. However, the smaller the grain size, the more easily the metal powder aggregates, which makes it difficult to achieve uniform dispersion in the conductive paste and also increases the contact area between adjacent metal particles to thereby increase the contact resistance, so that the conductivity cannot be sufficiently improved in accordance with an increase in packing ratio.
It is also known that traces having good conductivity can be obtained by using silver powder or Cu powder as the above metal powder.
However, when an electric field is applied to the conductive paste containing the silver powder in hot and humid atmosphere, electrodeposition of silver on an electrical circuit or an electrode, so-called migration, occurs to establish a short circuit between electrodes of the trace pattern or traces. As a countermeasure against the migration, for example, there has been known a method of applying a moistureproof coating onto the silver powder or a method of adding a corrosion inhibitor such as nitrogen oxide to the conductive paste, but they are not sufficiently effective (see Japanese Unexamined Patent Application Publication No. 2001-189107). In order to obtain a highly conductive conductor, moreover, it is necessary to increase the amount of the silver powder to be added, but the silver powder is so expensive that the electronic device also becomes expensive.
On the other hand, since Cu can easily be oxidized after heating and hardening the conductive paste containing the Cu powder, the Cu powder reacts with oxygen in the air or the binder to form an oxide film thereon, thereby remarkably reducing the conductivity. As its countermeasure, Japanese Unexamined Patent Application Publication No. 5-212579 discloses a Cu paste into which a variety of additives are added to prevent the oxidation of the Cu powder and stabilize the conductivity. However, it is inferior to the silver paste in conductivity and also has a defect in shelf life and stability.
A conductive paste using silver-plated Cu powder has also been suggested in order to realize an inexpensive conductive paste improved in the migration (see Japanese Unexamined Patent Application Publication Nos. 7-138549 & 10-134636). However, if the silver is applied uniformly and thick, the migration improving effect may not be obtained sufficiently. If it is applied thin, on the other hand, the filling amount of the conductive powder has to be increased for ensuring good conductivity, which may cause a problem of reducing the adhesive force (adhesive strength) due to relative decrease of the binder component.
Moreover, electronic devices for outdoor use, e.g., solar cells, need long-term durability against severe environmental changes. Particularly when solar cells are installed in desert having long sunshine duration or the like, the temperature range at the installation site may exceed 100° C. However, if conventional solar cells having their electrodes formed with the above technology are put in such a harsh natural environment, the electrodes may deteriorate in a few years to cause peeling or the like.